I tried to add a recipe in the FluidBlockTransformerRegistry file to transform an existing fluid in a barrel into another fluid by adding an item or block from my inventory and it's not working , instead of giving the fluid it just gives the missing texture , and I cant interact with the contents of the barrel once it's transformed.

I have a fork where I've added FluidItemFluid recipes using json of the form:

[
  {
    "inputFluid": "water",
    "reactant": {"name": "minecraft:redstone", "meta": 0},
    "output": "witchwater"
  }
]

If BloodWorkXGaming wants I can make a pull request.

My existing pull request now has the FluidItemFluid recipe type (I was already working on the same base).

This now exists as well thanks to @SirLyle !